staging: most: fix up movement of USB driver

When moving the most usb driver out of staging, we forgot to remove the
subdirectory out of the staging Makefile as well.

Fixes: 97a6f772 ("drivers: most: add USB adapter driver")
Cc: Christian Gromm <christian.gromm@microchip.com>
Reported-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200802092139.GA140088@kroah.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>